The invention relates to a hydraulic bearing according to the preamble of claim 1.
Hydraulically damped power unit bearings of the aforementioned kind should meet functional demands whose contradictory purposes require contradictory solutions accordingly. First of all, such power unit bearings are used to couple the power unit, especially of a motor vehicle, to the chassis. In the power unit""s static state of rest the static load of the power unit in the z-direction, which is perpendicular to the transverse and longitudinal extension of the motor vehicle, must be carried. Furthermore, when using an embodiment of the hydraulic bearing for a motor vehicle, the relative movement between the power unit and the chassis occurring under dynamic operational conditions must be limited. This requires high spring stiffness in all three coordinate directions. At the same time, such a hydraulic bearing is designed to isolate motor vibrations and therefore not to transmit these disturbing vibrations from the motor to the chassis. This requires a spring stiffness which is as low as possible. Since in common hydraulic bearings the supporting spring defines at the same time the boundary of a hydraulic working chamber, to be effective it should provide a large pumping area for the hydraulic fluid and a large volume stiffness.
These cross-purpose objectives may be summarized into two pairs of conflicts: The elastomer spring system of a hydraulic bearing used in a motor vehicle has to provide a large bearing displacement in the axial direction and at the same time a large extension path in the radial direction. Additionally, the elastomer spring system has to isolate resonances and disturbing vibrations and at the same time provide high dynamical long-lasting strength and volume stiffness.
According to the state of the art, several attempts are known to find a compromise between these cross-purpose objectives for the design of a supporting spring of a hydraulic bearing.
The publication of a German patent application DE 195 43 239 A1 describes weakening the supporting spring of the hydraulic bearing by means of recesses in the spring so that expansion areas are formed on the one hand. On the other hand the spring is smoothed in its spring behaviour to an extent that it meets the requirements for an isolation of vibrations. In order to satisfy the requirements regarding the static and dynamical load of the power unit, a supporting metal spring is located in the hydraulic working chamber. The metal spring limits the relative movement between the power unit and the chassis and supports the static and dynamical load of the power unit which rests on such bearings. Such a compromise, however, reduces the volume stiffness of the supporting spring, which is an essential feature for a sufficient dampening function of the bearing.
The compromise, according to the state of the art, lies in an extensionable elastomer membrane control spring which envelopes the metal spring by forming a control chamber. It can be hydraulically influenced and is located in the working chamber. The price paid by the state of the art is a complete additional control mechanism for the control spring correcting the spring characteristic of the radial spring. This is integrated with the expansion bladder. Another disadvantage is the high breakdown rate of the bearings caused by breakage of the metal spring due to unavoidable dynamical load.
In the light of this state of the art, it is an objective of the invention to create a hydraulic bearing enabling an improved balance between the bearing characteristics, the aims of which contradict each other. The invention solves this problem by a hydraulic bearing comprising the features of claim 1.
The basic concept of the invention is not only to constructively separate the axial supporting spring and the radial spring of the hydraulic bearing but also to constructively separate the expansion bladder and the radial spring. In contrast to the state of the art, the expansion bladder and the radial spring provide not only one but two elastomer bodies which are completely independent of one another. The expansion bladder is no longer formed by means of recesses located in the radial spring but by a rolling membrane as a second spring element separated from the radial spring. Due to this consequent splitting up of the common bearing body into an expansion bladder and a separate radial spring, both spring elements can be optimised independently of one another according to their specific requirements. The stiffness of the radial spring may be anisotropic for example. With respect to the helical supporting spring the stiffness relations between Z, X and Y may be adjusted to 10 to 1 to 1, respectively. Without changing the dimensions of the hydraulic bearing in general, other relationships of stiffness between Z, X and Y may be adjusted for example to 1 to 5 to 5, respectively. This is possible since there is no need for an elastomer bearing body to be extensible with respect to its configuration and dimensions even after the separation of the spring functions into a supporting spring and a radial spring. Moreover, both components, the radial spring as well as the separate expansion bladder, may be designed by choosing the stiffness and the quality of the used elastomer in accordance with the requirements of their respective applications. Other spring qualities, as for example temperature and the stability of the elastomer may only be realized when needed.
In top view of the hydraulic bearing in the direction from the load-side connection to the chassis-side connection the expansion bladder is located behind the radial spring inside the hydraulic bearing, forming a unique and exclusive membrane wall enclosing the hydraulic working chamber. In order to ensure a coordinative cooperation of the three spring elements, namely the supporting spring, the radial spring and the expansion bladder, the radial spring as well as the expansion bladder provide rigid coupling elements rigidly coupling and connecting the three spring components in a force-fit, a form-fit or a friction locked manner. According to an embodiment of the invention the radial spring and the expansion bladder provide inner metallic pieces. These inner metallic pieces are incorporated into the elastomer, being centrally located in axial direction and coaxial to one another and connected with each other in a fixed and rigid way by form-fit, force-fit, especially slug fit or frictional lock. Advantageously, a torsional safety device between the two inner pieces of the radial spring and the expansion bladder is formed, especially due to a seat configuration in the coupling area of the two inner pieces whose symmetry differs from a circle or due to tongue groove joint. In this way, the inner composite comprising the inner piece of the expansion bladder and the inner piece of the radial spring rests on the load-sided top of the helical supporting spring which is made of steel for example. This supporting spring directly or indirectly rests on the chassis-side connection of the hydraulic bearing. The term xe2x80x9cindirectlyxe2x80x9d denotes for resting of the supporting spring on intermediate elements of the hydraulic bearing, for example a common radial separating disc which itself rests on the real chassis or the chassis-side connection of the hydraulic bearing.
The known bearing body of the hydraulic bearing had to serve as a pumping area, an expansion bladder and at the same time as a supporting spring. Furthermore and also at the same time, it functioned as a radial spring. Due to the disintegration, a hydraulic bearing may be constructed Without a constructively greater effort and without a complicated additional control mechanism, whose spring characteristics are independently adjustable for each of the three principle axes as well as for each resulting axis according to the requirements of its application.
Due to these new constructional possibilities regarding the configuration of the hydraulic bearing, enabling even a relative smooth configuration of the radial spring at least in a predetermined direction, the helical steel spring which is used as a supporting spring may be subject to increasing radial strain. In order not to limit the design possibilities of the radial spring and not to accept restrictions of the configuration and dimensions of the supporting spring caused by compromise made, according to a further embodiment of the invention, the supporting spring only indirectly rests on the chassis-side connection of the hydraulic bearing in such a way, that it axially rests on an elastomer buffer or an elastomer thrust spring. The central spigot provided at the load-side of the thrust spring may have a centering effect with regard to the supporting spring resting on the thrust spring. Furthermore, the central spigot may act as a dampening stopper, provided the bearing is constructed accordingly. Said centering spigot is designed in correspondence to the inner contour of the helical spring used as a supporting spring and in such a way that it extends into the axial spring resting on it.
According to a further embodiment of the invention an additional stabilisation of the centering spigot formed at the thrust spring may be provided by vulcanizing a cup-shaped sheet metal into the centering spigot comprising an outer flange and opening towards the chassis-side.
Due to this design of the chassis-side support of the helical spring made of steel, share stresses may be effectively compensated. These share stresses cause well-known breakage of all kinds of helical springs rigidly supported at their chassis-side.
Therefore, it should be mentioned that said bearing or support of the supporting spring bearing, which has been described as an embodiment of the invention, may also be used in hydraulic bearings not being covered by the scope of disclosure.
Further developments of the inventions are subject matter of the dependent claims.